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A runner is halfway(13 miles) thru the marathon and sees that he has been running
for about 100 minutes. he realizes that in order to match his personal best he
will have to run faster. his personal best time for 26 miles is 191 minutes.
Q: what is the runner's rate (slope) during the second half of the race in minutes per mile?
Since he has used up 100 min in 1st 13 mi, he has to do the last 13 mi in 91 min
Speed = Dist/time
He has to run a rate of 7 min per mile
Check: 13 * 7 = 91 min